Firefly is a low-cost NASA/GSFC led nanosatellite mission, a collaborative effort sponsored by NSF (National Science Foundation). The overall objective is to study the relationship between lightning and TGFs (Terrestrial Gamma-ray Flashes) which are sudden (transient) energetic bursts in the upper atmosphere. The phenomenon of TGFs was first observed on NASA's CGRO (Compton Gamma Ray Observatory) mission in 1994 (CGRO launch on April 5, 1991 aboard the Space Shuttle Atlantis). A subsequent study from Stanford University in 1996 linked a TGF to an individual lightning strike occurring within a few ms of the TGF. 1)2)3)4)5)6)7)8)

The Firefly mission, funded and managed by NSF, will be developed as a collaborative effort by NASA/GSFC, USRA (Universities Space Research Association) of Columbia, MD; Siena College of Loudonville, NY; University of Maryland Eastern Shore, Princess Anne, MD; and the Hawk Institute for Space Sciences, in Pocomoke City, MD. NASA/GSFC, USRA and Siena College will provide the instrument payload, while the Hawk Institute will build the nanosatellite. NASA's Wallops Flight Facility on Wallops Island, VA, will provide technical oversight for the integration of Firefly to the launch vehicle.

TGFs are likely produced by beams of very energetic electrons, which are accelerated in the intense electric fields generated by large thunderstorm systems. These electron beams are more powerful than any produced in near-Earth space, and understanding their acceleration mechanisms will shed light on a physical process that may occur on other planets, or in astrophysical environments, as well as in the sun's corona.

The objective of Firefly is to explore which types of lightning produce these electron beams and associated TGFs. In addition, Firefly will study the occurrence rate of TGFs that are weaker than any previously studied. The goal is to examine the link between lightning and TGFs.

The nanosatellite, using a triple CubeSat configuration (3U), is being built by the Hawk Institute for Space Sciences (HISS), in Pocomoke City, MD. The 3U bus meets the P-POD standard. The spacecraft is attitude controlled via a gravity gradient boom to point within 20º of nadir, providing a good geometry for the communication antennas, good FOV (Field of View) for the GPS antenna in the zenith direction, unobstructed ground viewing of the subsatellite point by the optical photometers, and nadir viewing for the GRD (Gamma Ray Detector). The 3m gravity gradient boom stows into a small package, and when deployed, serves also as a monopole antenna element for the VLF radio receiver system. The GPS patch antenna is mounted on the end opposite the photodiodes.

The sides of the spacecraft are covered with either 6 or 7 (depending on the side) 28% UTJ (Ultra Triple Junction) large area solar cells.

The zenith side of the spacecraft (to the left of Figure 2) contains most of the “spacecraft bus” components, including the flight computer, radio, electrical power system, batteries, GPS receiver, GPS antenna, magnetometer, power switching, and experiment interface circuitry. The nadir side (to the right of Figure 2) contains the experiment boards (HV power supply, FPGA based experiment controllers, analog front ends for the sensors, scintillator crystal and PlanaconTM photon detector, gravity gradient / VLF boom, and photodiodes. The communication antennas and baluns are also on the nadir side of the spacecraft.

Launch: The Firefly nanosatellite was launched on Nov. 20, 2013 (01:15 UTC) as a secondary payload on the ORS-3 (Operationally Responsive Space-3) enabler launch mission of DoD. The launch vehicle was a Minotaur-1 of OSC (Orbital Sciences Corporation), and the launch site was MARS (Mid-Atlantic Regional Spaceport) on Wallops Island, VA. 9)10)

Note: The ELaNa-4 CubeSats were originally manifested on the Falcon-9 CRS-2 flight (launch of CRS-2 on March 1, 2013). However, when NASA received word that the P-PODs on CRS-2 needed to be de-manifested, NASA's LSP (Launch Services Program) immediately started looking for other opportunities to launch this complement of CubeSats as soon as possible. 11)12)

Orbit: Near-circular orbit, altitude ~ 500 km, inclination of =40.5º.

Secondary Payloads: The secondary payloads on this flight consist of 26 experiments comprised of free-flying systems and non-separating components (2 experiments). ORS-3 will employ CubeSat wafers, which enable secondary payloads to take advantage of excess lift capacity unavailable to the primary trial. 13)14)

The CubeStack adapter structure is a design by LoadPath and Moog CSA Engineering. 15)

Mission status:

• A NASA team made first contact with the National Science Foundation-funded Firefly spacecraft at 00:33 UTC on Jan. 7, 2014. On the first pass, the team – based out of NASA’s Wallops Flight Facility, Wallops Island, Va. – received enough data to show that the spacecraft was healthy and transmitting a strong signal. 16)

- The data volumes on the spacecraft had been filled, as expected, given that the spacecraft had been downloading data since launch on Nov 19, 2013. When the team downlinked the first installment of the data, they found that the spacecraft power system is healthy and the computer processing unit temperature is within a good range, at about 10ºC.

- The team will soon begin work to download the rest of the data, assess Firefly’s status, and then move the spacecraft into science mode. Firefly is led by a joint team of scientists from NASA's Goddard Space Flight Center in Greenbelt, Md., and Siena College in Loudonville, N.Y.

• Nov. 21, 2013: After attaining orbit, the Firefly deployer was activated at the correct time, and automatic beacons from other nanosatellites in the same deployment wafer have been detected. Firefly does not have an automatic beacon mode, however, so the team will attempt to make contact as it passes over Wallops on Nov. 21, 2013, following the release of updated orbital information from the DOD (Ref. 16).

The objectives are to provide snapshots, spectra, and count rate histograms. GRD will measure the energy and arrival time of X-ray and gamma-ray photons as well as the energetic electron flux.

The GRD consists of a 64 cm2 x 1 cm thick scintillator crystal (GYSO:Ce), arranged in a phoswich configuration with a 4 mm thick layer of PVT plastic scintillator (Eljen Technologies EJ-240). The GYSO:Ce has an optical decay time of 60 ns, and 20 photons / keV scintillation efficiency. The EJ-240 is a special, long-decay plastic scintillator, with an optical decay time of 285 ns and a scintillation efficiency of 4 photons / keV. Low-energy electrons (and low energy X-rays) tend to deposit all their energy in the front layer, which is the plastic scintillator.

Gamma rays (and high energy electrons) deposit their energy in the bulk scintillator (GYSO:Ce). The resulting light is collected by diffuse reflection inside the light-tight housing, and then detected by a Burle Planacon system, which combines a photocathode, microchannel plate, and anode in a hermetically sealed unit. The Planacon was chosen because of its mechanical robustness, insensitivity to humidity, and small form factor, coupled with large gain (up to 106).

The result is that the optical decay time of each pulse can be used to identify whether a given radiation quanta is an electron or photon. Figure 7 shows the result of lab calibrations of the GRD prototype. Plotted are the outputs of the fast and slow shaping amplifiers for each count that enters the detector. The blue dots are from a Co60 gamma source that emitted no electrons (due to attenuation in the source housing). The red dots are from a test in the GSFC electron beam facility, using 1.6 MeV electrons. At energies above ~100 keV (the plot is in raw ADC counts), there is a clean separation between these two populations, with very low false identification rates.

The incident photon or electron generates optical photons in the scintillator that are converted into charge via a photocathode / microchannel plate (MCP) pair in the Planacon. The pulses from the MCP are detected by a CSP (Charge Sensitive Preamplifier) and are then put through three separate filter chains.

- The first filter, called “Fast Shaper”, has a fast time constant (~300 ns) and will peak very rapidly. It will produce an output pulse with amplitude proportional to the energy content of the incoming photon. The output of the filter is then sent to a peak detector which will hold the peak of the pulse until the ADC (Analog Digital Converter) in the ECB converts it and a reset pulse is sent back to clear the peak detector.

- The second filter, called “Slow Shaper”, has a slow time constant (~1 µs). It processes incoming pulses similar to the “Fast Shaper” but it can be used for pulse-height analysis of both photons and electrons. Due to the slow response, it does not suffer from the ballistic deficit of the Fast Shaper. But its performance will be degraded by pulse pileup sooner than the fast filter. Species identification for incoming particles can be made by comparing the outputs of the two filters.

- The third filter has an extremely fast time constant. Its output is connected to two discriminators: an ULD (Upper Level Discriminator), and a LLD (Lower Level Discriminator). The two levels can be controlled by two I2C resistors. The output pulse of the LLD can be used to time the arrival of the particle in the detector. The output of the ULD is used to discard the pulse because the energy has exceeded the range of interest. The pulser, shown in Figure 6, is used for gain stabilization and dead time determination. It is a negative pulse with 1 kHz rate and 10% duty cycle that can be turned on and off by command.

OLD (Optical Lightning Detector):

The OLD detects the arrival time of the optical signal associated with lightning. The OLD as well as the VLF receiver are capable of time tagging the arrival of events to within 1 µs of onboard time.

The OLD is constructed by using four overlapping Hamamatsu silicon PIN photodiodes (S9195) which provide localization of lightning to one of twelve regions. Figure 8 shows the overlap of the OLD fields of view (FOV), and the twelve regions of overlap. The scale size of the region where all four fields overlap, at the subsatellite point, is approximately 100 km.

The 3 photodiodes will have visible/wide pass band filters, while the fourth will have a narrow band (5 nm wide) OI 777.4 nm spectral filter. The inclusion of the 777.4 nm filter will enable some discrimination of lightning and sprites, and will provide information on the lightning current moment waveforms. Students have been actively involved in the calculations for the field of view of OLD and the design of the field stop for each of the OLD detectors.

The ideal visible light lightning detector circuit would amplify only lightning signatures ignoring electronic noise, cloud crossings, terminator crossings and sun glints. In this circumstance, it is necessary to translate the optical signal to an electronic one and amplify it to a signal level of around +4.5 V so that it can be read by the 16 bit A/D converter.

Each photodetector has a FOV that extends to ±22.5º from the normal viewing angle. At the highest mission altitude of approximately 500 km, the furthest distance that we wish to see lightning is 500 km x cos (22.5º) = 541 km.

Given that the chosen photodetector (Hamamatsu's S9195) has an active area of 25 mm2 and an average response through the visible light filter of 0.36 A/W, the photodiode is expected to respond to lightning with a current of about 0.0245 µA which indicates that the output circuit needs to have a current to voltage transfer function of 1.84 x 108 V/A.

VLF (Very Low Frequency) receiver:

The VLF receiver is considered a secondary experiment, developed at Siena College, which will greatly expand the quality of the science for Firefly. The objective is to measure single-axis electric field signatures in the range of 100 Hz to ~ 1 MHz.

This instrument consists of a 3 m monopole BeCu antenna that will be deployed to measure electric field signatures in the range of about 500 Hz to 500 kHz using selectable filters set at 16 kHz, 32 kHz, and 500 kHz. Students at Siena College are developing modules in Matlab that will process the data received from the VLF receiver. Some of these algorithms include variable filters and FFTs (Fast Fourier Transforms).

Other student activities in support of Firefly include the analysis of the orbital parameters for the Firefly mission, the design of an inexpensive ground station, and the design of the Firefly mission logo.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.